1. Technical Field
The present invention relates to a nonvolatile memory device, and more particularly, to a reading method of a nonvolatile memory device.
2. Related Art
Semiconductor memory devices are generally classified into a volatile memory device and a nonvolatile memory device. The volatile memory device looses stored data when power is cut off, but the non-volatile memory device preserves stored data even when power is cut off. The nonvolatile memory device includes various types of memory cell transistors. The nonvolatile memory device can be divided into a flash memory device, a ferroelectric RAM (FRAM), a magnetic RAM (MRAM), a phase change RAM (PRAM), etc. according to the structure of the memory cell transistor.
The flash memory device is generally divided into a NOR flash memory device and a NAND flash memory device according to a connection state between a memory cell and a bit line. The NOR flash memory device has a structure in which two or more memory cell transistors are connected in parallel to one bit line. Thus, the NOR flash memory device has an excellent random access time characteristic. On the other hand, the NAND flash memory device has a structure in which two or more memory cell transistors are connected in series to one bit line. Such a structure is referred to as a cell string structure, and one bit line contact is required per cell string. Thus, the NAND flash memory device has an excellent characteristic in terms of a degree of integration.
The memory cells of the flash memory device are divided into an on-cell and an off-cell according to threshold voltage distributions. The on-cell is an erased cell, and the off-cell is a programmed cell. In order to read the memory cells of a flash memory device, a selection read voltage is applied to a selected memory cell, and a non-selection read voltage is applied to a non-selected memory cell.
The selection read voltage applied to the selected memory cell is a voltage for deciding an erase or program state of a selected memory cell. Therefore, if the selection read voltage is applied to a selected memory cell, cell current of the selected memory cell flows through a bit line based on a threshold voltage of the selected memory cell. The non-selection read voltage applied to a non-selected memory cell is a voltage sufficient to turn on the non-selected memory cell. Therefore, if the non-selection read voltage is applied to the non-selected memory cell, the cell current of the selected memory cell flows through the bit line without a decrease in cell current. The selection read voltage and the non-selection read voltage are generated from a voltage generator of the flash memory device. The selection read voltage and the non-selection read voltage are respectively applied to the selected memory cell and the non-selected memory cell without variations in voltage.